This invention is directed to charge-coupled devices, and more particularly, to a method of operating a charge-coupled device having a floating gate amplifier at the output.
Low noise detection and amplification of the signals represented by charge packets in charge-coupled devices (CCD's) are required so that CCD's may be used for low light level or other small signal applications. A non-destructive readout is also desirable to allow further processing of the signal after it is detected. The typical amplifier for detection of charge packets is the precharge amplifier in which a P-N junction is charged to a preset level through an MOS transistor and the signal charge is subsequently allowed to discharge the diode capacitance, thus producing a voltage difference proportional to the signal charge. Unfortunately, the precharge amplifier destroys the charge packet so that it cannot be further processed and produces a noise voltage proportional to .sqroot.kT/c due to thermal noise in the MOS transistor channel. A floating gate amplifier which provides a non-destructive readout has been described by Wen and Salsbury, "Analysis and Design of a Single Stage Floating Gate Amplifier", ISSCC Dig. Tech. Papers, February 1973, pp. 154-155 and Wen, "Design and Operation of a Floating Gate Amplifier", IEEE Journal of Solid-State Circuits, Vol. SC- 9, No. 6, December 1974, pp. 410-414. This device also eliminates the preset noise inherent in the preset amplifier. Its structure is such that the floating gate is embedded in an oxide layer underneath a larger bias gate which is used to transfer charge beneath the floating gate. The clocking of the bias gate to transfer charge packets introduces noise on the floating gate, and stresses the insulating oxide because of the large voltages impressed. An improved version of a capacitively coupled floating gate amplifier is disclosed in a copending application, "Capacitively Coupled Floating Gate Amplifier", by Joseph E. Hall, Ser. No. 21,058, filed Mar. 16, 1979. This improved version remotely locates the bias gate and uses control gates to transfer charge packets thereby improving device sensitivity and eliminating some of the noise on the floating gate. However, in all of these structures, there is no conductive connection to the floating gate. Control of the potential on the floating gate tends to be uncertain such that the potential exhibits a tendency to drift with time due to charge migration in the surrounding insulator. This drift changes the operating point of the amplifier and the CCD channel potential. A conductively coupled floating gate amplifier which gives excellent control of the floating gate potential is described in a copending application, "Floating Gate Amplifier Using Conductive Coupling For Charge Coupled Devices", by Charles G. Roberts and Joseph E. Hall, Ser. No. 45,466, filed June 4, 1979. This amplifier uses a reverse biased semi-conductor diode or an MOS transistor to couple the floating gate to a bias voltage. Although this amplifier gives excellent control of the floating gate potential, the clocking scheme used to transfer charge and set the voltage on the floating gate induces a significant amount of noise in the system and consequently reduces the sensitivity of the amplifier.